Intake and/or exhaust gas sensors may be operated to provide indications of various gas constituents. Output from an oxygen sensor, for example, may be used to determine the air-fuel ratio (AFR) of exhaust gas. An oxygen sensor may be disposed in an engine intake passage to determine the concentration of exhaust gas recirculation (EGR) gasses in intake charge air. Indications of AFR may be used to adjust various engine operating parameters such as fueling and a target AFR, for example. In particular, exhaust gas AFR may be controlled to achieve the target AFR in order to maximize operating efficiency of an emission control device. For some oxygen sensors, their output may significantly vary as a function of their operating temperature. As such, these oxygen sensors may be heated by a heating element to achieve a desired operating temperature range such that desired oxygen sensing is provided.
In some approaches, the impedance of an oxygen sensor is used to control the temperature of the oxygen sensor. For example, closed loop control may be employed to control the oxygen sensor temperature, where the sensor temperature is determined based on the impedance of an oxygen sensing element (e.g., a concentration cell) in the oxygen sensor.
The inventors herein have recognized that the impedance of such an oxygen sensing element can rise exponentially as the temperature of the sensing element decreases. As such, the impedance may be prohibitively high for determining the oxygen sensor temperature in certain temperature ranges.
Other factors pose challenges to oxygen sensor control. Thermal shock and cracking, for example, can occur in an oxygen sensor when heated while water is in contact with the sensor. High rates of heating, rapid increases to high rates of heating, and sustained heating particularly increase the incidence of such issues, which can degrade oxygen sensing and thus engine operation. Accordingly, some approaches to oxygen sensor control wait to heat an oxygen sensor until exhaust gas reaches a dew point temperature at which it is assumed that water in the exhaust system evaporates. Once the dew point temperature has been reached, the oxygen sensor temperature may be controlled via closed loop control, for example.
The inventors herein have recognized several issues with such an approach. Specifically, unevaporated water may remain in contact with the oxygen sensor upon reaching, or in some cases exceeding, the dew point temperature. This water may be puddled water accumulated on the oxygen sensor, mixed with exhaust gas, and/or generally present in the exhaust system, for example. Moreover, even with water fully evaporated off the oxygen sensor, additional water may subsequently impinge upon the sensor, for example in the event of water splash. If at this point the oxygen sensor temperature is controlled via closed loop control, a water splash will reduce the sensor temperature, prompting a rapid increase to high levels of heating by closed loop control, which may lead to thermal shock and cracking in the sensor.
Other approaches to oxygen sensor control attempt to actively detect water impingement on an oxygen sensor based on pumping current. Pumping current is the electrical current that results from electrochemically pumping a substance (e.g., oxygen) out of or into a concentration cell by applying a pumping voltage across the concentration cell (e.g., across two electrodes of the cell), and may be proportional to the substance within the concentration cell, yielding an indication of the concentration of the substance.
The inventors herein have recognized an issue with such an approach. Pumping current may be insufficient to detect water impingement on an oxygen sensor placed in an exhaust system. Specifically, the pumping current of such an oxygen sensor may remain around zero, since the concentration of oxygen in exhaust gas remains near zero during normal operating conditions, which is insufficient for water impingement detection. While in some approaches the pumping voltage is varied, this voltage is typically varied only for small durations under specific conditions, both of which are unsuitable for detecting water impingement.
One approach that at least partially addresses the above issues includes a method of operating an oxygen sensor comprising applying power to a heater of the oxygen sensor, and indicating whether water is in contact with the oxygen sensor based on a time rate of change of a temperature of the oxygen sensor.
In a more specific example, indicating whether water is in contact with the oxygen sensor includes indicating that water is in contact with the oxygen sensor responsive to the time rate of change being less than a minimum expected time rate of change of the temperature of the oxygen sensor expected for the power applied to the heater.
In another example, the method further comprises, prior to indicating whether water is in contact with the oxygen sensor, determining the temperature of the oxygen sensor based on only a resistance of the heater if the resistance indicates a temperature below or equal to a threshold temperature, and both the resistance of the heater and an impedance of the oxygen sensor if the resistance indicates a temperature above the threshold temperature.
In yet another example, the power is a first power level, and the method further comprises, responsive to indicating that water is in contact with the oxygen sensor, applying a second power level greater than the first power level to the heater, and determining whether one of an expected temperature and a minimum expected time rate of change of the temperature of the oxygen sensor is reached by the oxygen sensor, the expected temperature and the minimum expected time rate of change both expected for the second power level.
In this way, both the temperature of, and water impingement on, an oxygen sensor may be determined throughout its operational range, enabling appropriate actions that maintain desired oxygen sensor operation to be taken whether water contact on the sensor is detected or not. Thus, the technical result is achieved by these actions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.